Electric safety switch

ABSTRACT

A safety switch for an electric heater in which the heating element forms one arm of a balanced resistance bridge circuit. Electrical power for the element is applied across the bridge circuit through a pair of leads, each having an interrupting set of contacts. A fuse also connected across the bridge circuit is broken by current flowing therethrough when the element is broken or electrically shorted to unbalance the bridge. Breakage of the fuse mechanically releases a spring blade causing it to strike a plate opening both sets of contacts.

United States Patent [1 1 Vogel et al.

[ ELECTRIC SAFETY SWITCH [75] lnventors: John D. Vogel; Carl E. Bochmann;

Albert J. Parrigin, all of Columbus,

Ind.

[73] Assignee: Arvin Industries, Inc., Columbus,

Ind.

[22] Filed: Nov. 27, 1972 [21] Appl. No.: 309,945

[52] US. Cl. 219/499, 337/148 [51] Int. Cl. 1105b 1/02 [58] Field of Search 337/143, 148, 153, 337/156, 407; 219/253, 499, 517

[5 6] References Cited UNITED STATES PATENTS 2,615,107 10/1952 Zito 337/148 X 2,866,067 12/1958 Dolan et al. 219/499 Nov. 20, 1973 12/1959 Evans 219/499 11/1965 Peaton 219/499 Primary ExaminerBernard A. Gilheany Assistant ExaminerF. E. Bell AttorneyThomas P. Jenkins [5 7 ABSTRACT 16 Claims, 8 Drawing Figures SHEET 1 [F 2 PAIENIEU NOV 20 I975 Fig.3

Fig. 4

ELECTRIC SAFETY SWITCH BACKGROUND OF THE INVENTION Electric resistance heaters normally have an open faced metal housing which has its open face covered by a protective metal grill. A sheet metal reflector is mounted in the housing and an electric resistance heating element such as a helically wound wire, a flexible ribbon, or the like is suspended within the reflector on insulators. Should the element break or electrically short in any way, it could contact the metal grill, the metal reflector, or the metal housing and subject the user to a potential electrical shock.

Kipp U.S. Pat. No. 3,624,351 shows one way to minimize potential shock hazards to the user in the event the heating element breaks. In Kipp, the reflector is electrically insulated from the housing, and a second protective grill is secured to the reflector rearwardly of the outer or front grill. In this manner, a broken heating element is prevented from contacting any metal that is not insulated from the housing, and the shock hazard to the user is thereby minimized. This construction, however, requires the use of special insulator mountings to secure the reflector and the additional protective grill mounted on the reflector, all of which substantially increases the manufacturing cost. Moreover, this system safeguardsonly against breakage of the heating element. It does not protect against electrical shock caused by an accidental contact with the heating element, such asforexamplewhen ametal rod or the like is stuck through the grills and the heating element is shorted but notbroken It is therefore an object of this invention to provide an electric safety switch which will open'when the heating element either breaks or shorts, which can be simply and inexpensively manufactured and mounted inan electric heater, and which can be of a compact size and can be manufactured from a relatively small number of parts.

SUMMARY OF THE INVENTION In accordance with one form of the invention, asit is embodied in a combined thermostat and safety switch, an insulating base is provided with first and second metallic blades mounted thereon, and having first and second contacts engageable with each other. A bimetallic blade is mounted on said base and is engageable with said first blade to vary the relative position of said first and second contacts in response to changes in temperature. Third and fourth metallic blades having third and fourth normally closed contacts are also mounted on the insulating base.

A nonconductive switch plate is mounted on said first and third blades adjacent said first and third contacts and is adapted to bear against such blades to open said contacts. A fixed metallic blade and a metallic spring blade are also mounted on the insulating base. A fuse electrically couples the fixed and spring blades and also through the fuse causing it to break. when the fuse breaks, the spring blade strikes the switch plate and causes it to bear against and open both sets of contacts to open'the circuit to the heating element and thus prevent electrical shock to the user.

A rigid arm is desirably secured on the insulating base and is provided with outwardly projecting wings adapted to serveasmounts for mounting the switch in the heater. An adjustable screw ismounted on said arm and is-adapted to'engage said second blade to vary the position of the first and second contacts and thus provide for calibrated adjustment of the switch actuation temperature.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings illustrate the invention. In such drawings:

FIG. 1 is a vertical section through an electric heater having a switch embodying the invention mounted therein;

FIG. 2 is an enlarged side elevation of the switch shown in FIG. .1;

FIG. 3 is an enlarged side elevation of the switch shown in FIG. 2, but showing the contacts in an open DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT An electric safetyswitch 16 is mounted in an electric heater 10 as shown generally inFIG. l. The heater l0 comprises a metal housing 12 having an opening I3 in its front face 15 across which a metal grill 14 extends.

A reflector is mounted in the housing 12, and resistive heating element 20 is mounted in the reflector on insuheating element be electrically shorted in any way, as

by sticking a metal rod or the like through the grill and contacting but not necessarily breaking the heating element.

As shownin FIG. 2, the switch 16 comprises an insulating base 24 formed of a plurality of insulating blocks 26 fastened in a stacked position by a bolt 28. First and second metallic blades 30 and 32 are mounted at one of their ends on the base 24 and are electrically insulated from one another by one of the blocks 26. Terminals 34 and 36 are provided on the first and second blades 30 and 32 respectively for connecting them to a power line 33. First and second contacts 38 and 40 are mounted on the blades 30 and 32 adjacent their ends opposite the base 24, said contacts beingurged toward closed position by the inherent resiliency of said blades. 7

A bimetallic blade 42 is mounted at one of its ends to the base 24 and is electrically insulated from the first and second blades 30 and 32 by one of the insulating blocks 26. The end of blade 42 opposite the base 24 is bent to engage the blade 30 at its end adjacent the contact 38 to thus cause the contacts 38 and 40 to open or close in response to temperature changes.

A rigid arm 44 is mounted on the base 24 at one of its ends and extends outwardly from the base in the same direction as the blades 30, 32 and 42. The arm 44 is insulated from the first blade 30 by an insulating block 26 and includes suitable mounts for mounting the switch on a heater as shown in FIG. 1. The opening and closing of contacts 38 and 40 in response to the blade 42 is controlled by a temperature calibrated control assembly 29. Said assembly has an adjusting screw 46 threadably received in a boss 48 formed in the arm 44. The screw 46 has an insulator pin 50 at its inner end which extends through the arm 44 and an opening in the first blade 30 (not shown) to bear against the blades 32. If desired, the blade 32 can have an indentation 52 forming a seat for pin 50. The screw 46 extends outwardly through the front heater face 15 and is connected to a control knob 54 which is movable in front of suitable indicia (not shown) on the heater face to thus provide temperature calibrated adjustment of the actuation temperature of the contacts 38 and 40.

Third and fourth metallic blades 58 and 60 are also mounted on the base 24 and are insulated from the blade 42 and from each other by an adjacent pair of insulating blocks 26. Terminals 62 and 64 are mounted on the third and fourth blades 58 and 60, respectively, for connecting them to a power line 65. Third and fourth contacts 66 and 68 are mounted on the blades 58 and 60 adjacent their ends opposite the base 24 and are urged toward normally closed position by the inherent resiliency of said blades.

A generally rigid metallic blade 70 and a metallic spring blade 72 are also mounted on the base 24 and are electrically insulated from each other and from the blade 60 by a pair of adjacent insulating blocks 26, blade 72 being biased toward blade 60. As shown in FIG. 5, the ends of a fuse wire 78 are tied around tabs 86 and 88 struck inwardly from the blades 70 and 72 to electrically and mechanically interconnect said blades. The wire 78 is electrically coupled through terminals 74 and 76 on the blades 70 and 72 as will be hereinafter explained to the heating element 20 so that a current is caused to flow through the fuse wire 78 to break said wire whenever said element breaks or shorts.

A switch plate 80 of a nonconducting material is mounted on the blades 30 and 58 adjacent their respective contacts 38 and 66. When the element 20 breaks or electrically shorts, the excessive current on the wire 78 burns it in half thereby releasing the spring blade 72, as shown in FIG. 3. The spring blade 72 strikes the plate 80 which in turn bears against the blades 30 and 58 and moves them away from the blades 32 and 60 to open contacts 38, 40 and 66, 68. Thus, the heater is shut off to eliminate any potential shock hazard to the user.

FIG. 4 shows the configuration of the plate 80 and how it is mounted in bearing engagement on the first and third blades 30 and 58. Plate 80 has a generally rectangular shape and has rectangular slots 82 and 84 formed therein in which the ends of blades 30 and 58 are respectively received. The bimetallic blade 42 also extends through the slot 84, but it does not contact the plate 80 and is freely movable with respect to said plate. Thus, blade 42 is movable with respect to the plate and blade 30 is movable with and with respect to said plate. When the fuse wire 78 breaks as shown in 'FIG. 3, the spring blade 72 strikes the plate 80, as

shown by arrow 100 in FIGS. 3 and 4, and moves said plate against blades 30 and 58 to thereby cause the contacts to open, as shown in FIG. 3.

A modified embodiment of the fuse assembly is shown in FIG. 6. In this embodiment, aligned sets of notches 186 are formed in the spring blade 172 and the rigid blade 170. A fuse 178 in the form of a wire loop is tied around the blades 170 and 172 and again holds the spring blade under spring tension.

Still another embodiment of the fuse assembly is shown in FIG. 7. In this embodiment, aligned slots 286 are formed in the ends of the blades 270 and 272 and a link 278 in the form of a metal stamping is used to couple the blades and to hold the spring blade under spring tension. As shown, link 278 has a generally H- shaped configuration comprising a pair of transverse legs 279 received against the opposed faces of blades 270 and 272 and interconnected by a cross leg 280. The leg 280 has a web 281 of reduced cross section interrnediate its length which is easily meltable upon the fuse being subjected to an abnormal current load when the heating element 20 breaks or becomes shorted. As shown, pairs of fingers 282 angle outwardly from the blades 270 and 272 for holding the fuse on said blades.

The element 20 and switch are connected to the wheatstone bridge circuit 95 shown in FIG. 8. The element 20 is connected as one or more resistors 90, 91, 92 and 93 in the arms of the bridge circuit, and electrical power to operate said element is applied across the bridge by lines 33 and 65. The lead 33 is interrupted by the thermostatically controlled contacts 38 and 40, and the lead 65 is interrupted by the normally closed contacts 66 and 68. The fuse wire 78 is also connected R R X R and the current flowing through the fuse wire 78 is zero. Whenever any of the bridge resistances change, as when the element breaks or electrically shorts, an abnormal amount of current is caused to flow through the remaining resistors and also through the fuse wire 78. This causes the fuse wire 78 to break and thereby release the spring blade 72. The released spring blade bears agains plate forcing it to bear against the first and third blades 30 and 58 to open both sets of contacts 38, 40 and 66, 68, as shown in FIG. 3. Both sets of contacts 38, 40 and 66, 68 are opened in order to avoid possible shock hazards since either one of the lead lines 101 and 102 may be positive or ground connected.

As will be understood, when the embodiment in FIG. 6 or FIG. 7 is employed, the fuse 178 or 278 will be interconnected in the bridge circuit shown in FIG. 9 in the same manner as fuse 78.

We claim:

1. An electric switch for an electric heater having a resistive heating element, comprising an insulating base, first and second blades mounted on said base, first and second contacts mounted on said first and second blades and connected to a first power line, third and fourth blades mounted on said base, third and fourth normally closed contacts mounted on said third and fourth blades and connected to a second power line, a rigid blade mounted on said base, a spring blade mounted on said base, said heating element being connected to circuit means connected to said power lines, fuse means connected to said circuit means and adapted to break upon a predetermined current flow through said circuit means, said fuse means interconnecting said rigid and spring blades, and a plate operatively connected to said first and third blades, said spring blade bearing against said plate upon said fuse means breaking to cause said plate to move said first and third blades to open said first and second and said third and fourth contacts.

2. An electric switch as set forth in claim 1 in which said fuse means comprises a wire connected at its opposed ends to said rigid and spring blades.

3. An electric switch as set forth in claim 2 in which said rigid and spring blades have a pair of tabs, and said fuse means comprises a wire connected at its opposed ends to said tabs.

4. An electric switch as set forth in claim 1 in which said fuse means comprises a wire loop wrapped around said rigid and spring blades adjacent their ends remote from said base.

5. An electric switch as set forth in claim 1 in which each of said rigid and spring blades has a pair of notches formed therein adjacent its end remote from said base, and said'fuse means comprises a wire loop wrapped around said rigid and spring blades and received in said notches.

6. An electric switch as set forth in claim 1 in which fuse means comprises a metal stamping having a pair of legs received against the opposed faces of said rigid and spring blades and interconnected by a third leg.

7. An electric switch as set forth in claim 6 in which said third leg has a web'of reduced cross-section intermediate its length.

8. An electric switch as set forth in claim 1 in which said rigid and spring blades have aligned notches formed in their ends remote from said base, and said fuse means comprises a metal stamping having a pair of legs received against the opposed faces of said rigid and spring blades, and a third leg received in said notches and interconnecting said pair of legs.

9. An electric switch as set forth in claim 8 in which each of said rigid and spring blades has a pair of outwardly angled fingers disposed on the opposite sides of the notch formed therein.

10. An electric switch as set forth in claim 1 in which said first and third legs are received in a pair of slots formed in said plate, and said third blade is movable with said plate to open said third and fourth contacts and said first plate is movable with and with respect to said plate to open and close said first and second contacts.

11. An electric switch for an electric heater having a resistive heating element, comprising an insulating base, first and second blades mounted on said base, first and second contacts mounted on said first and second blades and connected to a first power line, third and fourth blades mounted on said base, third and fourth normally closed contacts mounted on said third and fourth blades and connected to a second power line, a rigid blade mounted on said base, a spring blade mounted on said base, bridge means coupled to said power lines, said heating element forming a part of said bridge means, fuse means coupled across said bridge means and adapted to break upon a predetermined cursaid heating element is in one leg thereof.

13. An electric switch for an electric heater having a resistive heating element, comprising an insulating base, first and second blades mounted on said base, first and second contacts mounted on said first and second blades and connected to a first power line, third and fourth blades mounted on said base, third and fourth normally closed contacts mounted on said third and fourth blades and connected to a second power line, a rigid blade mounted on said base, a spring blade mounted on said base, said heating element being connected to circuit means connected to said power lines, fuse means connected to said circuit means and adapted to break upon a predetermined current flow through saidcircuit means, said fuse means interconnecting said rigid and spring blades, a bimetallic blade mounted on said base and engageable with said first blade for opening and closing of said first and second contacts in response to temperature changes, manually manipulatable control means engageable with said second blade for adjusting the spacing between said first and second contacts, and a plate operatively connected to said first and third blades, said spring blade bearing against said plate upon said fuse means breaking to cause said plate to move said first and third blades to open said first and second and saidthird and fourth contacts.

14. An electric switch asset forth in claim 13 in which said bimetallic blade is movable with respect to said plate and said first blade is movable with and with respect to said plate.

15. An electric switch asset forth in claim 13 in which said control means is mounted on a rigid arm mounted on said base and having means for mounting said switch in said heater.

16. An electric switch for an electric heater having a resistive heating element comprising an insulating base, i a series of aligned first, second, third and fourth blades successively mounted on said base, first and second contacts mounted on said first and second blades and connected to a first power line, third and fourth normally closed contacts mounted on said base and connected to a second power line, a rigid blade mounted on said base, a spring blade mounted on said base between said fourth and rigid blade and biased toward said fourth blade, said heating element being connected to circuit means connected to said power lines, fuse means connected to said circuit means and adapted to break upon a predetermined current flow through said circuit means, said fuse means interconnecting said rigid and spring blades, and a plate operatively connected to said first and third blades, said spring blade bearing against said plate upon said fuse means breaking to cause said plate to move said first and third blades to open said first and second and said third and fourth contacts. 

1. An electric switch for an electric heater having a resistive heating element, comprising an insulating base, first and second blades mounted on said base, first and second contacts mounted on said first and second blades and connected to a first power line, third and fourth blades mounted on said base, third and fourth normally closed contacts mounted on said third and fourth blades and connected to a second power line, a rigid blade mounted on said base, a spring blade mounted on said base, said heating element being connected to circuit means connected to said power lines, fuse means connected to said circuit means and adapted to break upon a predetermined current flow through said circuit means, said fuse means interconnecting said rigid and spring blades, and a plate operatively connected to said first and third blades, said spring blade bearing against said plate upon said fuse means breaking to cause said plate to move said first and third blades to open said first and second and said third and fourth contacts.
 2. An electric switch as set forth in claim 1 in which said fuse means comprises a wire connected at its opposed ends to said rigid and spring blades.
 3. An electric switch as set forth in claim 2 in which said rigid and spring blades have a pair of tabs, and said fuse means comprises a wire connected at its opposed ends to said tabs.
 4. An electric switch as set forth in claim 1 in which said fuse means comprises a wire loop wrapped around said rigid and spring blades adjacent their ends remote from said base.
 5. An electric switch as set forth in claim 1 in which each of said rigid and spring blades has a pair of notches formed therein adjacent its end remote from said base, and said fuse means comprises a wire loop wrapped around said rigid and spring blades and received in said notches.
 6. An electric switch as set forth in claim 1 in which fuse means comprises a metal stamping having a pair of legs received agaiNst the opposed faces of said rigid and spring blades and interconnected by a third leg.
 7. An electric switch as set forth in claim 6 in which said third leg has a web of reduced cross-section intermediate its length.
 8. An electric switch as set forth in claim 1 in which said rigid and spring blades have aligned notches formed in their ends remote from said base, and said fuse means comprises a metal stamping having a pair of legs received against the opposed faces of said rigid and spring blades, and a third leg received in said notches and interconnecting said pair of legs.
 9. An electric switch as set forth in claim 8 in which each of said rigid and spring blades has a pair of outwardly angled fingers disposed on the opposite sides of the notch formed therein.
 10. An electric switch as set forth in claim 1 in which said first and third legs are received in a pair of slots formed in said plate, and said third blade is movable with said plate to open said third and fourth contacts and said first plate is movable with and with respect to said plate to open and close said first and second contacts.
 11. An electric switch for an electric heater having a resistive heating element, comprising an insulating base, first and second blades mounted on said base, first and second contacts mounted on said first and second blades and connected to a first power line, third and fourth blades mounted on said base, third and fourth normally closed contacts mounted on said third and fourth blades and connected to a second power line, a rigid blade mounted on said base, a spring blade mounted on said base, bridge means coupled to said power lines, said heating element forming a part of said bridge means, fuse means coupled across said bridge means and adapted to break upon a predetermined current flow through said bridge means, said fuse means interconnecting said rigid and spring blades, and a plate operatively connected to said first and third blades, said spring blade bearing against said plate upon said fuse means breaking to cause said plate to move said first and third blades to open said first and second and said third and fourth contacts.
 12. An electric switch as set forth in claim 11 in which said bridge means is a wheatstone bridge and said heating element is in one leg thereof.
 13. An electric switch for an electric heater having a resistive heating element, comprising an insulating base, first and second blades mounted on said base, first and second contacts mounted on said first and second blades and connected to a first power line, third and fourth blades mounted on said base, third and fourth normally closed contacts mounted on said third and fourth blades and connected to a second power line, a rigid blade mounted on said base, a spring blade mounted on said base, said heating element being connected to circuit means connected to said power lines, fuse means connected to said circuit means and adapted to break upon a predetermined current flow through said circuit means, said fuse means interconnecting said rigid and spring blades, a bimetallic blade mounted on said base and engageable with said first blade for opening and closing of said first and second contacts in response to temperature changes, manually manipulatable control means engageable with said second blade for adjusting the spacing between said first and second contacts, and a plate operatively connected to said first and third blades, said spring blade bearing against said plate upon said fuse means breaking to cause said plate to move said first and third blades to open said first and second and said third and fourth contacts.
 14. An electric switch as set forth in claim 13 in which said bimetallic blade is movable with respect to said plate and said first blade is movable with and with respect to said plate.
 15. An electric switch as set forth in claim 13 in which said control means is mounted on a rigid arm mounted on said base and having means for mounting said switch in said heaTer.
 16. An electric switch for an electric heater having a resistive heating element comprising an insulating base, a series of aligned first, second, third and fourth blades successively mounted on said base, first and second contacts mounted on said first and second blades and connected to a first power line, third and fourth normally closed contacts mounted on said base and connected to a second power line, a rigid blade mounted on said base, a spring blade mounted on said base between said fourth and rigid blade and biased toward said fourth blade, said heating element being connected to circuit means connected to said power lines, fuse means connected to said circuit means and adapted to break upon a predetermined current flow through said circuit means, said fuse means interconnecting said rigid and spring blades, and a plate operatively connected to said first and third blades, said spring blade bearing against said plate upon said fuse means breaking to cause said plate to move said first and third blades to open said first and second and said third and fourth contacts. 